Serum thymidine kinase 1 activity as a pharmacodynamic marker of cyclin-dependent kinase 4/6 inhibition in patients with early-stage breast cancer receiving neoadjuvant palbociclib

Breast Cancer Res. 2017 Nov 21;19(1):123. doi: 10.1186/s13058-017-0913-7.


Background: Thymidine kinase 1 (TK1) is a cell cycle-regulated enzyme with peak expression in the S phase during DNA synthesis, and it is an attractive biomarker of cell proliferation. Serum TK1 activity has demonstrated prognostic value in patients with early-stage breast cancer. Because cyclin-dependent kinase 4/6 (CDK4/6) inhibitors prevent G1/S transition, we hypothesized that serum TK1 could be a biomarker for CDK4/6 inhibitors. We examined the drug-induced change in serum TK1 as well as its correlation with change in tumor Ki-67 levels in patients enrolled in the NeoPalAna trial ( identifier NCT01723774).

Methods: Patients with clinical stage II/III estrogen receptor-positive (ER+)/HER2-negative breast cancer enrolled in the NeoPalAna trial received an initial 4 weeks of anastrozole, followed by palbociclib on cycle 1, day 1 (C1D1) for four 28-day cycles, unless C1D15 tumor Ki-67 was > 10%, in which case patients went off study owing to inadequate response. Surgery occurred following 3-5 weeks of washout from the last dose of palbociclib, except in eight patients who received palbociclib (cycle 5) continuously until surgery. Serum TK1 activity was determined at baseline, C1D1, C1D15, and time of surgery, and we found that it was correlated with tumor Ki-67 and TK1 messenger RNA (mRNA) levels.

Results: Despite a significant drop in tumor Ki-67 with anastrozole monotherapy, there was no statistically significant change in TK1 activity. However, a striking reduction in TK1 activity was observed 2 weeks after initiation of palbociclib (C1D15), which then rose significantly with palbociclib washout. At C1D15, TK1 activity was below the detection limit (<20 DiviTum units per liter Du/L) in 92% of patients, indicating a profound effect of palbociclib. There was high concordance, at 89.8% (95% CI: 79.2% - 96.2%), between changes in serum TK1 and tumor Ki-67 in the same direction from C1D1 to C1D15 and from C1D15 to surgery time points. The sensitivity and specificity for the tumor Ki-67-based response by palbociclib-induced decrease in serum TK1 were 94.1% (95% CI 86.2% - 100%) and 84% (95% CI 69.6% -98.4%), respectively. The κ-statistic was 0.76 (p < 0.001) between TK1 and Ki-67, indicating substantial agreement.

Conclusions: Serum TK1 activity is a promising pharmacodynamic marker of palbociclib in ER+ breast cancer, and its value in predicting response to CDK4/6 inhibitors warrants further investigation.

Trial registration:, NCT01723774. Registered on 6 November 2012.

Keywords: Anastrozole; Biomarker; Breast cancer; Neoadjuvant; Palbociclib; Thymidine kinase.

MeSH terms

  • Adult
  • Aged
  • Antineoplastic Agents / pharmacokinetics*
  • Antineoplastic Agents / therapeutic use
  • Antineoplastic Combined Chemotherapy Protocols / therapeutic use
  • Biomarkers
  • Biomarkers, Tumor
  • Breast Neoplasms / blood*
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / pathology
  • Combined Modality Therapy
  • Cyclin-Dependent Kinase 4 / antagonists & inhibitors*
  • Cyclin-Dependent Kinase 6 / antagonists & inhibitors*
  • Dose-Response Relationship, Drug
  • Enzyme Activation / drug effects
  • Female
  • Humans
  • Middle Aged
  • Molecular Targeted Therapy
  • Neoadjuvant Therapy
  • Neoplasm Grading
  • Neoplasm Staging
  • Piperazines / pharmacokinetics*
  • Piperazines / therapeutic use
  • Protein Kinase Inhibitors / pharmacokinetics*
  • Protein Kinase Inhibitors / therapeutic use
  • Pyridines / pharmacokinetics*
  • Pyridines / therapeutic use
  • Thymidine Kinase / blood*


  • Antineoplastic Agents
  • Biomarkers
  • Biomarkers, Tumor
  • Piperazines
  • Protein Kinase Inhibitors
  • Pyridines
  • Thymidine Kinase
  • thymidine kinase 1
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinase 6
  • palbociclib

Associated data